In recent years, liquid crystal displays are widely used in a variety of fields, such as AV (Audio Visual) and OA (Office Automation) fields. In particular, liquid crystal displays of the passive type, which use TN (Twisted Nematic) and STN (Super Twisted Nematic) liquid crystal, are installed in those products of lower price. Further, liquid crystal displays of the active-matrix driving system, which use TFTs (Thin Film Transistors), that is, three-terminal non-linear elements, as switching elements, are installed in those products of higher price.
The liquid crystal displays of the active-matrix driving system have features that are superior to those of CRTs (Cathode Ray Tubes) in color reproducibility, thinness, light-weight and low power consumption, and the application of these displays has been rapidly expanding. However, the use of TFTs as switching elements require thin-film forming processes and photolithography processes of 6-8 times or more, resulting in high production costs. In contrast, liquid crystal displays using two-terminal non-linear elements as switching elements are less expensive to produce compared with those using TFTs and also exhibit superior display quality compared with those of the passive type. Therefore, the use of these displays has been rapidly developing.
As shown in FIG. 6, a liquid crystal display using the two-terminal non-linear elements has a display panel 1 wherein signal electrode lines X.sub.1.sup..about. X.sub.m and scanning electrode lines Y.sub.1.sup..about. Y.sub.m are disposed in a matrix form, in the same manner as a general liquid crystal display. To the signal electrode lines X.sub.1.sup..about. X.sub.n, are applied predetermined voltages, that correspond to display data and which are released by a signal-electrode driving circuit 2 based on control signals from a control section 4. To the scanning electrode lines Y.sub.1.sup..about. Y.sub.m, are applied predetermined voltages that are released by a scanning-electrode driving circuit 3 in a line-sequential manner based on control signals from the control section 4.
Further, as shown in FIG. 7, a liquid crystal element 5 and two-terminal non-linear element (hereinafter, referred to as two-terminal element) 6, which are connected in series with each other, are installed in each pixel that is formed at each intersection of the signal electrode lines X.sub.1.sup..about. X.sub.n and scanning electrode lines Y.sub.1.sup..about. Y.sub.m.
In general, the characteristic of the two-terminal element 6 is represented by an I-V (current versus voltage) characteristic that is indicated by a solid line shown in FIG. 10. More specifically, this characteristic exhibits a minute current with a high equivalent resistance when the applied voltage of the two-terminal element 6 is low, and also exhibits an abruptly increased current with a low equivalent resistance when the applied voltage of the two-terminal element 6 is high. Therefore, this characteristic is utilized when a displaying operation is carried out by using the two-terminal element 6.
In other words, when a displaying operation is carried out, a voltage that allows the liquid crystal element 5 to turn on is applied thereto by applying high voltage to the two-terminal element 6 so that it has low-resistance. In contrast, in the case of an operation with no display, a voltage that makes the liquid crystal element 5 turn off is applied thereto by applying a low voltage to the two-terminal element 6 so that it has high-resistance.
Moreover, the voltage which has been applied to the liquid crystal element 5 during a selection period, is maintained since the two-terminal element 6 becomes high-resistive during a non-selection period. Therefore, it is possible to provide a high-duty driving operation in a display using the two-terminal element 6, compared with a simple-matrix display.
However, in the two-terminal element 6, the initial characteristic, as described above, varies with the applied voltage and time; this causes a problem wherein an afterimage phenomenon (also referred to as seizure phenomenon) occurs; that is, the present display is influenced by the previous displaying state.
This afterimage phenomenon is caused by the time dependence of the applied voltage in the I-V characteristic of the two-terminal element 6. In other words, as shown in FIG. 10, the I-V characteristic of the two-terminal element 6 Shifts from the state indicated by a solid line to the state indicated by a broken line as the voltage-applying time increases. For this reason, as shown in FIG. 11, a V-T (Voltage versus Transmittance) characteristic of the liquid crystal element 5 also shifts from the state indicated by a solid line to the state indicated by a broken line. At this time, for example, a voltage which provides a transmittance of 50% shifts from V.sub.50 to V.sub.50'. Here, the amount of shift differs depending on the applied voltage.
As a result, as shown in FIG. 12, the amount of shift (indicated by a solid line), which allows the liquid crystal element 5 to turn on, becomes greater than the amount of shift (indicated by a broken line) for turning the liquid crystal element 5 off, as the voltage-applying time increases. The increase in the difference of the amounts of shift causes adverse effects such as afterimages and seizures in the display.
Here, there have been proposed various manufacturing processes and structures of the two-terminal element 6, which can eliminate the above-mentioned shift in characteristic, as well as driving methods, which can eliminate the influence of shift in characteristic of the display state.
For example, Japanese Laid-Open Patent Publication No. 29748/1996 (Tokukaihei 8-29748) discloses a driving method wherein the selection period during which the scanning electrodes are selected is divided into two periods and wherein afterimage phenomenon is reduced irrespective of display states by applying a sufficient voltage during the first half of the period.
In a matrix-type display using liquid crystal and other materials, when a certain pattern (black portion) is displayed, a pattern (shaded portion), which is not related to the display information, tends to appear along an extended line of the displayed line, as shown in FIG. 13. This phenomenon, called crosstalk, arises mainly from the following two problems: one is due to rounding in waveforms that are caused by wiring resistances and parasitic capacities of the signal electrodes. The other is due to the fact that effective voltages, which are applied to the display elements, are fluctuated by influence of data signals during the non-selection period in the so-called duty driving operation which uses methods such as the voltage-averaging method that is well known as a driving method for simple-matrix-type liquid crystal displays.
In order to solve the former problem, the following countermeasures have been proposed by modifying the manufacturing processes and designs of the display panel: low resistance materials are used as electrode resistances; electrode resistances are modified so as to have a stacked-layer wiring structure; the wiring shape is modified; etc.
In the case when the two-terminal element 6 is used, it is possible to provide displays in high quality because its characteristic allows the voltage, which has been applied to liquid crystal during the selection period, to be maintained even during the non-selection period. However, in this case, although less influence is caused compared with the simple-matrix system, such as STN, crosstalk tends to occur due to the latter problem, since the influence of data signals during the non-selection period is not eliminated completely.
Referring to FIGS. 14 and 15, the following description will discuss the way crosstalk is generated. Here, for convenience of explanation, FIG. 14 shows a display state on a display panel wherein the number of pixels is eight per one line. More specifically, three display states are shown: (A) all pixels are turned on; (B) every other pixel is turned on; and (C) only one selected pixel is turned on. Further, the following description deals with only one frame portion of the frame inversion in the voltage-averaging method. Since it is easily assumed that the same effects would be obtained from one-line inversion and multiple-line inversion as long as display data are synchronous to the inversion cycle, the descriptions of those inversions are omitted.
In the above-mentioned display states of A through C, voltage waveforms, which are to be applied to the respective selected pixels, are indicated by A.sub.3.sup..about. C.sub.3 in FIGS. 15(a) through 15(c). In each of FIGS. 15(a) through 15(c), a rectangular waveform portion, indicated by a solid line S, represents a waveform of voltage that is composed of a voltage applied by the signal electrode and a voltage applied by the scanning electrode, and a shaded portion represents a waveform of a voltage that is to be applied to a display element (liquid crystal in this case) through the non-linear element.
FIGS. 15(a) through 15(c) indicate that the effective values of the voltages that are to be applied to the respective selected pixels, A.sub.3.sup..about. C.sub.3, are represented by A.sub.3 &gt;B.sub.3 &gt;C.sub.3 since they are equivalent to the above-mentioned shaded portions and they are therefore different from one another. Moreover, since the transmittance of liquid crystal is dependent on the effective value of voltage, the selected pixels are displayed in black as shown in FIG. 14, for example, in the case when the display mode is set to normal white. With respect to the darkness of the displays, A is the darkest and C is the least dark. Also, with respect to the darkness of the displays of non-selection pixels, C is the least dark.
As shown in FIGS. 16(a) through 16(c), when the driving method of Japanese Laid-Open Patent Publication No. 29748/1996 (Tokukaihei 8-29748) is adopted, crosstalk can be reduced since the influence of data during the non-selection period is reduced to half, compared with the case shown in FIGS. 15(a) through 15(c), as indicated by A.sub.4.sup..about. C.sub.4 (shaded portions) of applied voltage waveforms to the respective selected pixels in the display states of A through C. However, since there are still slight differences among the effective voltages that are to be applied to the pixels in the above-mentioned three display states, crosstalk is not completely eliminated. This causes a problem of degradation in display quality when the large-size panel with high duty is used for displaying and when gradational displays are made.
With respect to driving methods that provide for crosstalk prevention in liquid crystal displays using non-linear elements, the following three methods are listed:
In a driving method disclosed in Japanese Examined Patent Publication No. 6210/1987 (Tokukoushou 62-6210), the selection period has both the first period during which the scanning signal is set at the selected level and the second period during which the scanning signal is set at the non-selected level. In this driving method, the driving level is set so that, during the first period, the display signal has a level corresponding to image information and so that, during the second period, it has a level inverted to that of the first period.
Further, in a driving method which is disclosed in Japanese Examined Patent Publication No. 64875/1991 (Tokukouhei 3-64875) and which is applied to the case where signal polarities are inverted at every horizontal period, the selection period has the first period during which the scanning signal is formed into a selection-level signal and the second period during which the scanning signal is formed into a non-selection-level signal. In this driving method, the driving level is set so that the display signal is formed into level signals that are inverted between the selected and non-selected states depending on the first and second periods. More specifically, the display signal is formed into a selection or non-selection level signal that corresponds to image information during the first period. Then during the second period, the display signal is formed into a non-selection level signal when it was a selection level signal during the first period, and is formed into a selection level signal when it was a non-selection level signal during the first period.
Moreover, in a driving method disclosed in Japanese Examined Patent Publication No. 49712/1992 (Tokukouhei 4-49712), which is applied to the case of a two-frame ac system, the influence of data during the non-selection period is reduced by using virtually the same methods as the above-mentioned two driving methods.
The use of either of the above-mentioned driving methods is supposed to sufficiently reduce crosstalk caused by the influence of data during the non-selection period, since the variation of effective voltage that is to be applied to pixels can be suppressed.
However, the above-mentioned three driving methods fail to prevent afterimages, and in terms of display quality such as contrast, they can only achieve characteristics that are the same as those obtained by conventional commonly-used driving methods. Therefore, the problem of the above-mentioned driving methods is that the characteristics of non-linear elements cannot be fully utilized.